Utd players hit the ground to stop play. That clearly indicated that they thought setpieces were their only chance.
Many of the fouls on City players were late but as the ball had been retained, it was presumably deemed advantage. One or two of the advantages were minimal at best, and you could see that some of the late hits were annoying the players - Walker definitely got one that he was annoyed about. I assume that it was drilled into the players to keep calm and ignore the ref's behaviour as much as possible.